The present invention relates to electrical machines which convert mechanical energy into electrical energy, or vice versa, by an interaction between a magnetic field and an electric current. In particular, the present invention relates to a casing for such an electric machine, a rotor for such an electric machine, a conductive element of a rotor winding for such an electric machine, and a method of forming such a conductive element. Examples of such electric machines are electric motors, dynamos and alternators.
In known electric machines, the assembly of a rotor winding may be time consuming and therefore expensive. Moreover the rotor winding often comprises a large number of different parts, which increases the overall cost of the electric machine.
The performance of such electric machines may also be limited by the amount of heat generated in the rotor winding and in the region of the commutator. As a result of this limited performance, the range of applications of such electric machines, particularly in electrically powered vehicles, has been restricted.
According to one aspect of the present invention, there is provided a conductive element of an armature for an electrical machine, comprising a metal strip having a pair of leg portions joined together at or about one end by a flat bridging portion, the first and second leg portions being bent in opposite directions perpendicularly to the plane of the bridging portion.
The armature may be easily constructed by arranging a number of such conductive elements with even circumferential spacing around a circle, to form the structure of the armature.
Preferably the first and second leg portions have a substantially equal width so that the assembled rotor consists of two winding planes of equal thickness, perpendicular to the axis of rotation.
However, in an alternative embodiment each conductive element may have three leg portions joined together at or about one end to a bridging portion, the two outer leg portions on either side of the bridging portion being bent in the same direction perpendicularly to the plane of bridging portion, and the middle leg portion being bent in an opposite direction.
In this alternative embodiment, the middle leg portion preferably has twice the width of each of the outer leg portions, so that the armature, when assembled, has three winding planes perpendicular to the axis of rotation, the middle winding plane having twice the thickness of each of the outer winding planes. This construction reduces shearing between the winding planes at high rotation speed and therefore reduces the risk of damage to the armature.
Preferably, each of the leg portions includes a radial portion, in which the current carried in the winding interacts with an applied magnetic field, and an outer portion which is bent towards the tangential direction of the armature so that it may be joined to another conductive element displaced around the circumference of the armature.
Preferably, the above-mentioned conductive element is stamped from metal sheet and the first and second leg portions are bent in opposite directions perpendicular to the metal sheet. In the method of forming the conductive element having three leg portions, the middle leg portion is bent in one direction perpendicular to the metal sheet, while the outer leg portions are bent in an opposite direction.
According to another aspect of the present invention, there is provided a conductive armature for an electrical machine, having a current-carrying winding formed from a plurality of integrally formed conductive elements circumferentially distributed around the armature, in which the radially outer portions of adjacent conductive elements have a gap between them to allow cooling fluid to flow through the radially outer portion of the armature. Each conductive element has a radially outer portion bent towards the tangential direction of the armature, the gap between adjacent conductive elements extending along a substantial portion of the length of the radially outer portions.
Adjacent ones of the conductive portions abut against each other in a radially inner area of the radially outer portions. Thus, cooling fluid is contained within the radially outer portion of the conductive elements, thereby enhancing cooling in the radially outer portions.
According to another aspect of the present invention, there is provided a conductive armature for an electrical machine, having a current-carrying winding comprising a plurality of circumferentially distributed integrally formed conductive elements, in which the surfaces providing the commutator are edge surfaces of the integrally formed conductive elements, adjacent ones of the conductive elements being spaced apart in the commutator area to allow cooling fluid to flow between the conductive elements in that area. As a result, greater cooling can be achieved in the commutator area, and brush dust, insulator and other debris are removed by the flow of cooling fluid.
Preferably, the major surfaces of the conductive elements in the commutator area are coated with an insulating material which is brittle or has relatively low wear resistance. As a result, electrical contact between adjacent conductive elements in the commutator area, caused for example by conductive brush dust, is prevented, while the insulating coating is worn down by contact with brushes which contact the commutator, in order to maintain a good contact between the brushes and the commutator. The insulating material which is worn away, together with brush dust and other debris, may then be removed by the cooling fluid flowing through the spaces between the conductive elements in the commutator area.
According to a further aspect of the present invention, there is provided a casing for an electrical machine having a rotor, the casing having cooling apertures to allow cooling fluid to flow into said casing, through the rotor and out of the casing when the rotor rotates, at least some of the apertures being louvres located in the radially outer portion of the casing, the louvres being inclined to direct out of the casing cooling fluid which circulates in the casing when the rotor rotates. Others of the apertures may be also located in a radially outer portion of the casing and may be louvres inclined to direct cooling fluid into the casing when the rotor rotates. In this way, the flow of cooling fluid is driven through the casing by the action of the rotor, so that the rotor is self-cooled. Moreover, the cooling fluid flow is driven when the rotor rotates in either direction.
Preferably, the louvres inclined in a first sense are arranged in a first plane perpendicular to the axis of rotation of the rotor, while the louvres inclined in a second, opposite sense are arranged in a second plane parallel to the first plane, so that cooling fluid is directed through the casing and through the rotor with an axial component when the rotor rotates.
Alternatively, apertures may be disposed in a radially inner portion of the casing, whilst louvres are disposed in a radially outer portion of the casing, arranged to direct air out of the casing when the rotor rotates. Thus, air is drawn into the apertures and directed out of the louvres, the flow of cooling fluid from the apertures to the louvres being assisted by the centrifugal force on the fluid circulating within the casing.
Preferably, the casing may include a fluid passage having an inlet arranged on the opposite side of the rotor from the apertures, and an outlet arranged to direct cooling fluid onto a radially outer portion of the rotor. Thus, the cooling fluid flows through the rotor from the apertures to the inlet of the fluid passage and is directed in an axial direction at a radially outer portion of the rotor towards the louvres. In this embodiment, the action of the louvres reduces the pressure of cooling fluid within the casing, thereby drawing cooling fluid into the apertures at the radially inner portion. Because the louvres are inclined in one sense only, this embodiment is only effective in one direction of rotation of the rotor.
Preferably, the rotor for use with the casing has gaps at its radially inner and radially outer portions which permit cooling fluid to flow axially through the rotor at these portions. Preferably, the rotor is a disc rotor with either the radially inner or radially outer portions of either or both of the faces providing the commutator, so that the cooling fluid flows through the rotor in the commutator area.
According to another aspect of the present invention, there is provided a rotor for an electrical machine, having a current-carrying winding comprising a plurality of circumferentially distributed conductive elements each having ends which lie at a radially outer region of the rotor, interconnections between said ends being made by interconnecting members having vanes arranged to direct cooling fluid axially across said ends.
According to another aspect of the present invention, there is provided an electric machine comprising a current-carrying rotor and a stator for producing an axial magnetic field through the rotor, the axial spacing between the rotor and the stator being variable, so as to vary the magnetic field in the rotor.
Preferably the stator comprises first and second sets of permanent magnets arranged on opposite sides axially of the rotor, the first and second sets of permanent magnets being supported respectively on first and second support members coupled to move in opposite axial directions relative to the rotor.
The separation between the first and second support members may be varied by camming means arranged between the members, or by one or more screw threaded rods engaging the first and second support members.